Pentacyclic and tetracyclic terpenoids represent a group of natural substances—isoprenoids, showing a large range of biological activities (Dzubak, P.; Hajduch, M.; Vydra, D.; Hustova, A.; Kvasnica, M.; Biedermann, D.; Markova, L.; Urban, M.; Sarek, J. Nat. Prod. Rep. 2006, 23, 394-411), thanks to which they are getting into the focus of the pharmaceutical industry. However, neither modified nor semisynthetic derivatives of these natural substances possess optimum pharmacological properties. Among their disadvantages belong namely low solubility in water-based media and further disadvantageous pharmacokinetic indicators such as low biological availability, short half-time of excretion and insufficient stability, which are unsuitable for carrying out in vivo tests in animals as well as for subsequent use in treatment of patients.
The pentacyclic and tetracyclic terpenoids are almost water-insoluble, since they have rigid lipophilic skeleton, composed of 25-30 carbon atoms, even if they bear polar functional groups such as —OH, —COOH, ═O, —NH2 etc. The solvents commonly used in the chemical practice, such as chloroform, acetone, ethyl acetate etc., cannot be used for dissolving for pharmaceutical purposes, for the reason of their incompatibility with living organisms. In the art it is known that in the presence of alkali carbonates or hydrogencarbonates, pentacyclic triterpenoid acids form inclusion compounds with cyclodextrins, these inclusion compounds being soluble in water-based media with the addition of suitable additives (WO 92/09553). It is taught that the highest solubility of triterpenoid acids can be achieved with the use of higher cyclodextrins, namely β and γ, and lower alcohols (methanol) or glycols (propylene glycol, butandiol) are used as additives (Uekama K., Hirayama F., Irie T.: Chem. Rev. 1998, 98, 2045-2076, Hedges A. R.: Chem. Rev. 1998, 98, 2035-2044). In the vehicles used, the triterpenoids reach the solubility between 10 and 50 mg/ml (WO 92/09553). The inclusion compounds can be isolated from their solutions as solid substances in the form of powder by means of lyophilization. However, only native triterpenoid carboxylic acids having a free carboxylic functional group can be dissolved in water-based media, but not their functional derivatives or terpenoids that do not have carboxylic functional group. Furthermore, in the pharmaceutical practice, the biologically active free triterpenoid acids, having many disadvantageous pharmacological properties, e.g. difficult purificability and instability, are often converted into derivatives bringing often a slower metabolization (increase of the half-life), increase of stability or functioning as prodrug. One type of these derivatives are various biologically cleavable esters, such as e.g. morpholinoethyl esters, acetoxymethyl esters, heptyl esters etc. (Gewehr M., Kunz H.: Synthesis 1997, 1499; Urban M., Sarek J., Tislerova I., Dzubak P., Hajduch M.: Bioorg. Med. Chem. 2005, 13, 5527)
In general, the carboxylic acid derivatives are even less compatible with water-based vehicles than the free acids. For the exploitation of the carboxylic acid derivatives in the pharmaceutical practice, it is necessary to find a formulation enabling their use with the water-based vehicles.
From the above given reasons it is clear that for further development, it is necessary to prepare the derivatives of the insoluble biologically active pentacyclic and tetracyclic terpenoids that are soluble in water-based media, are bioavailable (preferably orally available), have a suitable half-time of excretion and are stable, i.e. that have optimum pharmacokinetic parameters.